Optical lens systems do not generally have their best focus on a planar surface. For example, spherical lens systems tend to best focus on a roughly hemispherical surface, called the Petzval surface. Much of the complexity of lens design is in forcing the lens system to achieve best focus on a planar imaging surface, far away from the Petzval surface.
Zoom lenses introduce additional difficulty because the surface of best focus changes as a function of focal length. Because of this, zoom lenses are generally significantly less sharp than fixed focal length, or prime, lenses.